User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment, between a user equipment and a wire connected telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, or base stations, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
The 3GPP is responsible for the standardization of GSM, UMTS, LTE and LTE-Advanced. LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative UMTS.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the radio network node.
Discontinuous Reception (DRX) is a mechanism defined in 3GPP specifications for LTE to enable power savings of involved user equipments and conserve battery power. With this feature, user equipments may disable its receiver most of the time and just listen to the downlink channels during wake time, discontinuously. Discontinuous reception is applicable both in Radio Resource Control (RRC) idle state (RRC_IDLE), i.e. when the user equipment is in an idle state, and in RRC_CONNECTED states, i.e. when the user equipment is connected with the radio network node for data transmission and/or signalling.
One of the targets of the on-going enhancements for Diverse Data Applications (eDDA) in 3GPP is to provide long periods of always-on connectivity for user equipments without emptying the battery when using diverse data applications. In other words, discontinuous reception is an important feature as it helps user equipment to save battery and prolong user equipment activity time. It is of course good to configure a long discontinuous reception cycle for user equipment from power saving point of view. However long discontinuous reception cycle will result in long delay, it is not good from performance point of view. This problem is more severe when handover is taken into consideration, in particular when hard handover is utilized, as is the case in for example LTE, as the handover of the user equipment may fail, and the connection with the serving node get lost, under certain circumstances. A hard handover may be referred to in LTE when there is a short interruption in service when the handover is performed.
Specifically, user equipment with long discontinuous reception cycle takes longer time to trigger a handover event, since user equipment may only measure downlink signalling quality within wake intervals during each discontinuous reception cycle. This is illustrated by the timing of the measurements in FIG. 1, wherein signal quality is shown on the y-axis and time is shown on the x-axis. As further illustrated in FIG. 1, when the used discontinuous reception cycle is long, e.g. 1.28 s, the user equipment sends a measurement report later than expected. At time T2, the neighbour cell is already A3 offset better than the serving cell, however, not until time T3, may the user equipment know this event and not until time T5 after Time To Trigger (TTT) period, may the user equipment may send the measurement report to the radio network node, or eNB. Hence, the handover might not be able to finish in time, or even cannot finish before the link quality becomes too poor to work, i.e., a Radio Link failure may happen before that.
Therefore, a problem addressed herein is how to reduce the handover delay while keeping the power consumption of an involved user equipment low.
Since discontinuous reception aims at user equipment power consumption, user equipment measurement would only happen within on-duration of discontinuous reception cycles. A straightforward solution within the scope of 3GPP specification is to configure a very short TTT length, so that the user equipment would send the measurement report to the eNB as soon as possible when the user equipment knows that an A3 event threshold is satisfied. However this method has several problems. First, this method may only reduce the handover delay after the A3 event is detected by user equipment. However, the delay from the time when the neighbour cell is A3 offset, in e.g. dB, better than serving cell to the time when user equipment detect this event cannot be reduced, i.e. this method may only reduce the delay between T3 and T5 but cannot reduce the delay between T2 and T3 as shown in FIG. 1. Furthermore, setting the TTT to be very short would result in quite many unnecessary measurement reports from the user equipment to the eNB, resulting in unnecessary and inefficient use of resources.
Another prior art proposal is to use a short discontinuous reception cycle when the user equipment send a measurement report to the eNB so that user equipment may receive handover command quickly and the user equipment restore to use long discontinuous reception cycle if eNB does not initiate handover procedure. This method may help to reduce the handover delay after time t5 as shown in FIG. 1, but does not help to reduce the delay from t2 to t5 which is more important and may easily cause radio link failure or handover failure.